High-power DC voltage electrical systems are undergoing significant development. Indeed, numerous transport systems include a DC voltage supply.
Hybrid combustion/electric or electric vehicles include, in particular, high-power batteries. To obtain the appropriate voltage level, several electrochemical accumulators are placed in series. To obtain high powers and capacities, several groups of accumulators are placed in series. The number of stages (number of groups of accumulators) and the number of accumulators in parallel in each stage vary as a function of the desired voltage, current and capacity of the battery. The association of several accumulators is called a “battery of accumulators.” The electrochemical accumulators used for such vehicles are generally of the lithium ion type for their capacity to store significant energy with a confined weight and volume. Battery technologies of lithium-ion iron phosphate LiFePO4 type form the subject of significant developments on account of a high intrinsic safety level, to the detriment of a somewhat restrained energy storage density.
Such batteries are used to propel an AC electric motor by way of an inverter. The voltage levels necessary for such motors reach several hundreds of volts, typically of the order of 400 volts. Such batteries also comprise a high capacity so as to favor the autonomy of the vehicle in electric mode. Several technical reasons specific to the automobile application lead to the use of insulation between the vehicle's mechanical mass (formed by the vehicle's metal chassis and metal body-shell, and therefore accessible to the user) and the potentials of the battery. The main reason is that it is not conceivable upon a first insulation defect while driving to instantaneously disconnect the traction battery. For example, in the case where one of the poles of the battery is linked to the mechanics while the insulation defect appears on the other pole. This is manifested by a short-circuit and the immediate fusing of the protection fuse. This would have the effect of rendering the vehicle dangerous. On account of the disappearance of traction power or of recuperative braking, this therefore makes it obligatory to insulate the battery and to check this insulation for reasons of personal safety with an insulation monitor. Indeed, if upon a first defect there is no risk for the user, it is appropriate to alert him of this first defect before the appearance of a second defect having the effect of disconnecting the traction battery since this causes a short-circuit between the positive and negative terminals of the battery. Moreover, upon this second defect, the voltage of the battery would be directly linked to the vehicle's mechanical mass and the user would therefore be potentially in contact with the latter. On account of the potential risk of such an energy source for users, the insulation and the monitoring of the insulation between the battery and the mechanical mass must be particularly meticulous. Any conducting part of the vehicle must be insulated with respect to the masses. This insulation is effected through the use of insulating materials. The insulation may deteriorate with time (because of vibrations, mechanical knocks, dust, etc.), and therefore connect the mechanical mass to a dangerous potential.
Moreover, it is conceivable to use a charger that is not galvanically insulated from the electrical network. The mechanical mass of the vehicle being normatively grounded during recharges and the neutral regime used conventionally (EE regime) residentially connecting the neutral to the earth, this amounts to connecting the mechanical mass of the vehicle to one of the potentials of the battery during recharges. During these recharges, the complete voltage of the battery is therefore applied across the terminals of the insulation in contradistinction to the nominal case where only half this voltage is applied and above all monitored. This insulation might not be capable of coping with the complete voltage creating a second defect instantaneously resulting in a short-circuit.
An electric vehicle according to the prior art typically exhibits a battery intended for the power supply of a three-phase electric motor. The battery comprises electrochemical accumulators. A protection device furnished with fuses is connected to the terminals of the battery. An insulation monitoring device is also connected to the terminals of the battery and linked up to the mechanical mass of the vehicle. The insulation monitoring device is connected to a computer to signal the insulation defects detected to it. This computer is powered by an onboard network battery. The terminals of the battery apply voltages +Vbat and −Vbat to the DC inputs of an inverter by way of a cutoff system. The cutoff system comprises power contactors controlled by the computer. The electric motor is connected to the AC output of the inverter. Various types of insulation monitoring are known from the prior art.
Document FR2671190 describes, in particular, a device for monitoring insulation of a DC voltage electrical network. This document describes a resistive bridge injecting an AC component (about 30 V) at low frequency (between 4 and 10 Hz). A detection circuit measures a current passing through an insulation impedance and a measurement resistor down to ground. The design of such a circuit involves a compromise in the rating of the resistors of the resistive bridge.
The resistive bridge induces an electrical consumption that remains relatively significant so as to retain appropriate measurement precision. Such current consumption may turn out to be incompatible with an application in onboard systems, for example on account of the decrease in autonomy of an electric vehicle. Moreover, such a device is relatively expensive in particular on account of the use of a low-frequency generator rated for a high DC voltage. Furthermore, the detection circuit allows only the detection of an insulation defect between one of the terminals and ground, but not the detection of an insulation defect between the other terminal and ground. Moreover, such a monitoring device is sensitive to false positives which are manifested by untimely detections since it detects an insulation defect when AC currents pass through common-mode capacitors present in the inverter.
In another solution usually implemented in an electric vehicle 1, the insulation monitoring device comprises a resistive voltage divider. A bidirectional optocoupler is connected between the midpoint of the voltage divider and the mechanical mass. The resistors of the voltage divider on either side of the midpoint are identical. Thus, in the absence of an insulation defect, the voltage across the terminals of the optocoupler is zero and no insulation defect is signaled. Upon the appearance of an insulation defect between one of the terminals of the battery and the mechanical mass, the potential of the midpoint of the voltage divider is shifted. A voltage then appears across the terminals of the optocoupler, thereby generating an insulation defect signal.
With regulations imposing a relatively low threshold of leakage current to be detected, the resistors present in the voltage divider must exhibit a relatively restricted value, on the order of 50 kΩ. These resistors then induce a relatively significant DC electrical consumption to the detriment of the autonomy provided by the battery.
The chart of FIG. 14 illustrates the leakage current for a minimum battery voltage (in this instance of 192 V) as a function of insulation resistance, for various values of resistances between the terminals and the midpoint. It is noted that a value of the resistances of 10 kΩ makes it possible for example to detect insulation defect resistances of less than 50 kΩ, while a value of the resistances of 56 kΩ makes it possible to detect insulation defect resistances of less than only 1 kΩ.
Furthermore, on account of aging, certain insulation materials may turn out to be appropriate to withstand the voltage between the mechanical mass and a terminal of the battery under normal operation, but may blow when they are subjected to the total voltage between the terminals of the battery on account of an insulation defect. Such a monitoring device does not make it possible to test and to detect a potential insulation defect such as this, and this may lead to chain insulation defects. A first insulation defect on one polarity applies the total voltage between the other polarity and the ground. If the insulation of this other polarity was not able to withstand it, the second insulation defect appears. This creates a short-circuit with fusing of the fuses. This corresponds to a loss of traction and then sudden immobilization of the vehicle, which is dangerous.
Moreover, such an insulation monitoring device makes it possible to detect the insulation defect, but not to determine its magnitude.